The act of suspension is hanging the human body from (or partially from) hooks pierced through the flesh in various places around the body.
There are many different reasons to suspend, from pure adrenaline or endorphin rush, to conquering ones fears, to trying to reach a new level of spiritual consciousness and everything in between. In general, people suspend to attain some sort of "experience".
Some people are seeking the opportunity to discover a deeper sense of themselves and to challenge pre-determined belief systems which may not be true. Some are seeking a rite of passage or a spiritual encounter to let go of the fear of not being whole or complete inside their body.
Others are looking for control over their body, or seek to prove to themselves that they are more than their bodies, or are not their bodies at all. Others simply seek to explore the unknown.